Hi Trailblazer, that is the common West Bend/US Motor Power crankshaft size. Jim Donovan at Max Torque will build you one up in a day and ship it. Probably cost you about $95. Different than his four stroke clutches internally with the fiber washers. I have one and it has been working great for a few years now.
Or, as Carplay said, Jack Shaft clutch it. Comet Kart sales is having a closeout sale on the drive gears at ten bucks each. Just be mindful that you might want to run a bit stiffer on the clutch spring for higher RPM engagement than the standard 4 stroke clutch spring brings to the table.
Edit: Just went out and checked mine, and the PTO was externally threaded. You could tap the shaft you have easily to accept a retaining bolt. Also there are a couple of crankshafts on Ebay. Just wanted to point out that you do have options.